1. Field of the Invention
This invention relates to the art of electrochemical cells, and more particularly to a new carbonaceous material that is useful as a negative electrode in a rechargeable, secondary electrochemical cell. 2. Prior Art
Carbonaceous materials are known for use as electrodes in electrochemical cells due to their high conductivity and gross pore structure. Carbon also exhibits a high capacity for the intercalation and deintercalation of alkali metal ions over extended cycles. This has resulted in carbon electrodes being used extensively in rechargeable, secondary cells.
U.S. Pat. Nos. 5,028,500 and 5,069,683 to Fong et al. describe the use of a carbonaceous electrode in a rechargeable, electrochemical cell comprising an alkali metal, a first electrode intercalatable with the alkali metal, a counterelectrode capable of reversibly incorporating the alkali metal, and a suitable electrolyte. The carbonaceous electrode preferably serves as the counterelectrode and comprises a particulate composition including carbon, such as graphite or coke, and an electrically conductive filamentary material, such as carbon black interspersed with the carbon-containing composition. In one embodiment of a reversible electrochemical cell, the alkali metal is initially positioned in an electrically connected and layered relationship with the carbonaceous electrode serving as the counterelectrode or anode. The alkali metal, such as lithium, intercalates into the carbonaceous electrode in a lithiation process to saturate the anode and the cell then discharges in a normal manner.
A second embodiment of this secondary cell comprises the carbonaceous electrode serving as the anode and the cathode having lithium incorporated therein. The cell is in a discharged state when first assembled so that applying an externally generated electrical potential recharges the cell by drawing lithium from the cathode material and through the electrolyte to intercalate into the carbonaceous anode.
U.S. Pat. No. 5,030,529 to Wada et al. discloses a carbon electrode for rechargeable electrochemical cells comprising first and second carbon layers. The first carbon layer is described as containing an iron-family element, such as nickel. The first carbon layer is formed by vapor pyrolytic deposition of a hydrocarbon compound such as propane, benzene and the like, at about 400.degree. C. to 1,300.degree. C. to deposit carbon onto either a nickel substrate or on a fine nickel powder. The second carbon layer is free from any iron-family elements and is formed over the surface of the first layer by using vapor deposition with a higher concentration of evaporated hydrocarbon to increase the deposition rate.
There is thus needed a new electrode material that exhibits a higher capacity for the intercalation of alkali metal ions than previously known carbonaceous electrode materials and which is characterized by excellent reversibility for the intercalation and deintercalation of the alkali metal ions over extended discharge and recharge cycles.